Ring laser gyros use ring lasers whose laser path is usually imbedded in a dimensionally stable block of glass-ceramic material. Usually a closed gas-filled bore is fabricated with three or more, typically four, corner mirrors to circulate counterpropagating laser beams around a closed path. Means are provided to deliver energy to the laser. As the laser is turned around an axis enclosed by the laser path, the Sagnac effect causes the two counterpropagating laser beams to vary in frequency. The difference in frequency between the beams is a measure of the angular velocity of the turning about that axis.
To determine the difference in frequency between the two beams, one of the corner mirrors transmits a part of both beams through its transparent substrate. At the data of this writing, the amount of transmitted beam is on the order to 0.01% of the beam's energy. An optical combining system is used to combine the two beams to obtain moving interference fringes that may be counted. The rate of the counter is then a measure of the angular velocity of the ring laser about its sensing axis, while the direction of fringe motion indicates the sense of rotation.
In the prior art, parts of the two counterpropagating beams are also extracted at a second corner mirror, and the intensities of the extracted laser beams are monitored and used to servo the inward and outward motion of one or more of the other corner mirrors to tune the laser cavity.
Because of the extra energy escaping from the ring laser when light is extracted at two of its corner mirrors, the finesse of the ring laser cavity is reduced. Reduction of the finesse of the ring laser reduces its accuracy as a measuring instrument.
In many situations, it is important to keep the bulk of the ring laser instrument as small as possible.
It is common to dither ring laser gyros around their sensing axis. When the optical beam combiner extends outward from the ring laser, because of the increased moment of inertia more energy is expended in dithering the gyro. Further, the combiner acts like a pendulum, and its pendulosity unbalances the ring laser. More counter-weights must be added or more mass must be removed to balance the ring laser. Larger counter-weights increase the moment of inertia and more energy is required for the dithering. A larger enclosing structure is needed for the outwardly extending combiner.
When combining mirrors or prisms for ring laser gyros produce different path lengths for the two counterpropagating beams, a significant phase shift occurs in the sensed fringe information, over ambient temperature changes, thereby introducing an error into the instrument. The claimed apparatus herein has a beam-splitting mirror and a glue line of finite but very small thickness. That the path lengths thus are slightly different for the two beams, and that difference is temperature sensitive, but very small.